首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献，搜索用时 62 毫秒
1.
The role of the fluid phase during regional metamorphism and deformation   总被引：8，自引：0，他引：8  
Evidence from rock microstructures, mass transfer and isotopic exchange indicates that substantial quantities of aqueous fluids are involved in low- and medium-grade regional metamorphism. Similar conclusions are drawn from many retrograde environments, whereas high-grade metamorphic fluids may be melt dominated. The mobile fluids play essential roles in metamorphic reactions, mass transport and deformation processes. These processes are linked by the mechanical consequences of metamorphic fluid pressures (Pf) generally being greater than or equal to the minimum principal compressive stress. Under such conditions metamorphic porosity comprises grain boundary tubules and bubbles together with continuously generated (and healed) microfractures. Deformation results in significant interconnected porosity and hence enhanced permeability. Lithologically and structurally controlled permeability variations may cause effective fluid channelling.
Simple Rayleigh-Darcy modelling of a uniformly permeable, crustal slab shows that convective instability of metamorphic fluid is expected at the permeabilities suggested for the high Pf metamorphic conditions. Complex, large-scale convective cells operating in overpressured, but capped systems may provide a satisfactory explanation for the large fluid/rock ratios and extensive mass transport demonstrated for many low- and medium-grade metamorphic environments. Such large-scale fluid circulation may have important consequences for heat transfer in and the thermal evolution of metamorphic belts.  相似文献   

2.
Creation of pathways for melt to migrate from its source is the necessary first step for transport of magma to the upper crust. To test the role of different dehydration‐melting reactions in the development of permeability during partial melting and deformation in the crust, we experimentally deformed two common crustal rock types. A muscovite‐biotite metapelite and a biotite gneiss were deformed at conditions below, at and above their fluid‐absent solidus. For the metapelite, temperatures ranged between 650 and 800 °C at Pc=700 MPa to investigate the muscovite‐dehydration melting reaction. For the biotite gneiss, temperatures ranged between 850 and 950 °C at Pc=1000 MPa to explore biotite dehydration‐melting under lower crustal conditions. Deformation for both sets of experiments was performed at the same strain rate (ε.) 1.37×10?5 s?1. In the presence of deformation, the positive ΔV and associated high dilational strain of the muscovite dehydration‐melting reaction produces an increase in melt pore pressure with partial melting of the metapelite. In contrast, the biotite dehydration‐melting reaction is not associated with a large dilational strain and during deformation and partial melting of the biotite gneiss melt pore pressure builds more gradually. Due to the different rates in pore pressure increase, melt‐enhanced deformation microstructures reflect the different dehydration melting reactions themselves. Permeability development in the two rocks differs because grain boundaries control melt distribution to a greater extent in the gneiss. Muscovite‐dehydration melting may develop melt pathways at low melt fractions due to a larger volume of melt, in comparison with biotite‐dehydration melting, generated at the solidus. This may be a viable physical mechanism in which rapid melt segregation from a metapelitic source rock can occur. Alternatively, the results from the gneiss experiments suggest continual draining of biotite‐derived magma from the lower crust with melt migration paths controlled by structural anisotropies in the protolith.  相似文献   

3.
A series of striking migmatitic structures occur in rectilinear networks through western Fiordland, New Zealand, involving, for the most part, narrow anorthositic dykes that cut hornblende‐bearing orthogneiss. Adjacent to the dykes, host rocks show patchy, spatially restricted recrystallization and dehydration on a decimetre‐scale to garnet granulite. Although there is general agreement that the migration of silicate melt has formed at least parts of the structures, there is disagreement on the role of silicate melt in dehydrating the host rock. A variety of causal processes have been inferred, including metasomatism due to the ingress of a carbonic, mantle‐derived fluid; hornblende‐breakdown leading to water release and limited partial melting of host rocks; and dehydration induced by volatile scavenging by a migrating silicate melt. Variability in dyke assemblage, together with the correlation between dehydration structures and host rock silica content, are inconsistent with macroscopic metasomatism, and best match open system behaviour involving volatile scavenging by a migrating trondhjemitic liquid.  相似文献   

4.
低渗透储层油、水两相渗透率特征及其影响因素的研究,是认识、分析低渗透储层油、水两相渗流机理的重要途径,是合理开发油田的基础。本文在总结低渗透油藏油水相对渗透率曲线特征的基础上,对以火山岩、凝灰岩岩屑为主的岩屑砂岩相对渗透率曲线特征进行研究,并分析了其影响因素,研究表明低渗透储层的岩石学特征,碎屑颗粒的成份、结构,其表面的粗糙程度,及其粘土矿物的产状对相对渗透率曲线有明显的影响。这一研究成果对合理开发此类低渗透油田有着十分重要的意义。  相似文献   

5.
砂岩侵入体的形成机制分析*   总被引：1，自引：1，他引：0       下载免费PDF全文
砂岩侵入体是指深水沉积砂体受到外界的触发,并在一定条件下形成超压,致使上覆弱渗透性沉积物等围岩发生破裂,砂体以流化和液化的形式向周围沉积物产生侵入。砂岩侵入体的形成过程为形成超压、盖层破裂、产生液化和流化、发生侵入。差异压实、地震引发的液化、流体的加入和压力传递等多种因素都可以使砂岩中形成超压,当地层压力达到并超过破裂压力时,发生水力破裂,或地震引起上覆地层破裂,超压砂岩发生流化,侵入到低渗透围岩中。大量未固结的深水沉积砂岩、低-非渗透层的快速覆盖、形成超压的机制和触发事件是砂岩发生侵入的必备条件。  相似文献   

6.
Compressional waves velocity VP was measured during long-term experiments in a high-pressure vessel (in the range [10–75] MPa for confining and pore pressures). Experiments were carried out on a granite specimen prepared by a controlled heating treatment at 510 °C, which generated thermal cracks.

Data analysis is proposed by using an effective medium approach based on Kachanov's [Appl. Mech. Rev. 45 (1992)304] model. The elastic behaviour of the cracked rock is controlled by the crack density parameter which varies with confining and pore pressures due to crack closure. In order to model the progressive closure of cracks, we assume elliptical cracks with major axis 2c and aspect ratio . By using a conformal mapping technique, we derive the variation of the crack aspect ratio as a function of effective pressure, the effective pressure coefficient η depending on and Poisson's ratio ν0. As a result, we compute the crack density parameter and the elastic moduli of the cracked rock as a function of confining and pore pressures. To take into account the heterogeneity of the rock sample, a peak-like distribution of crack aspect ratios is introduced, which allows us to calculate the acoustic velocity VP for various effective pressures.

Comparison is made between theoretical and experimental values and shows that this simple model captures the essential features of the acoustic velocity variation: an increase of VP when pore pressure is decreased followed by a plateau for a threshold pore pressure. Best consistency between theoretical and experimental velocity values is obtained by introducing a second crack population with a higher mean aspect ratio and an irreversible closure mechanism as effective pressure is cycled.  相似文献   


7.
地下盐岩溶腔是CO2封存的有效地质体,CO2沿盐岩软弱夹层和盐层-夹层交界面泄漏是制约地下盐岩溶腔CO2安全封存的关键。以苏北盆地金坛地区CO2盐穴储气库为研究对象,建立了层状盐穴储气库CO2封存的流-固耦合数学模型,分析了盐岩及泥岩夹层中CO2运移泄漏规律及其对CO2安全封存的影响,并探讨了盐岩及泥岩夹层渗透率的动态响应特征。结果表明：渗透率是决定盐岩层中CO2运移速率和泄漏范围的关键,在其影响下,相同封存时间内泥岩夹层中CO2运移速率和影响范围远大于盐岩,但随封存时间延长,盐岩和泥岩夹层中CO2运移速率和压力增幅均呈降低趋势,并随着CO2压力传播至模拟边界而趋于稳定。渗透率动态变化是上覆地层压力负效应与盐岩层中CO2压力正效应共同作用的结果,并受盐岩和泥岩夹层力学性质的影响。CO2封存时间<3 ...  相似文献   

8.
Fluid is released by dehydration reactions during prograde metamorphism. If the Claperyron slope for the dehydrating reaction is positive, then there is a net decrease in the total solid volume, which implies an irreversible increase in porosity. If the dilation of the pore space is insufficient to provide storage for all the released fluid, then pore pressure excess is generated, and if it becomes sufficiently high, it may lead to brittle fracturing. The time scale for pressure generation and the pore pressure excess can be maintained over long duration hinge on the interplay of reaction kinetics and fluid drainage. Motivated by experimental and microstructural observations, a hydrological model is developed that incorporates dehydration kinetics and its pressure dependence. Analytic solutions were derived for the undrained development of pore pressure. Whether lithostatic pressure may be exceeded hinges on magnitude of the overstep in temperature and corresponding equilibrium pressure. The time scale for development of pore pressure depends on the trade-off between poroelasticity and the pressure sensitivity of reaction rate. A finite difference model was also developed to simulate the progressive development of pore pressure excess, dehydration and porosity development. The model captures the experimental observation in gypsum of a reaction front that progressively propagates from the drained end toward the undrained end of a laboratory sample. It is also in reasonable agreement with experimental data on fluid drainage and porosity production.  相似文献   

9.
Reasonable determination of formation fracturing pressure concerns the stable operation of underground fluid injection projects. In this work, we studied the effect of unsteady flow on fracturing pressure. Hydraulic fracturing tests on low permeable sandstone were conducted with the injection rate between 0.1 and 2.0 ml/min. Then, the fracturing pressure prediction models for hollow cylinder under both unsteady flow and steady flow conditions were deduced. Finally, the effect of unsteady flow on the fracturing pressure was studied based on the experimental result and several influence factors. It was shown that fracturing pressure increased with the elevated pressurization rate in the tests, while the slope of the variation curve decreases. The model considering unsteady flow can reflect the variation tendency of fracturing pressures in experiments, while fracturing pressures from the model considering steady flow are invariant with different pressurization rates. Fracturing pressure decreases with the elevated rock permeability and increases with the elevated fluid viscosity, and these two effects are actually generated by the unsteady flow. Whether to consider the unsteady flow has no significant influence on the effect of rock tensile strength on fracturing pressure when the tensile strength is very low. However, when the tensile strength is high, the effect of unsteady flow cannot be neglected.  相似文献   

10.
Microcracks in brittle rocks affect not only the local mechanical properties, but also the poroelastic behavior and permeability. A continuum coupled hydro-mechanical modeling approach is presented using a two-scale conceptual model representing realistic rock material containing micro-fractures. This approach combines a microcrack-based continuous damage model within generalized Biot poroelasticity, in which the tensors of macroscopic elastic stiffness, Biot effective stress coefficient and of overall permeability are directly related to microcrack growth. Heterogeneity in both mechanical and hydraulic properties evolves from an initially random distribution of damage to produce localized failure and fluid transmission. A significant advantage of the approach is the ability to accurately predict the evolution of realistic fracturing and associated fluid flow in permeable rocks where pre-existing fractures exert significant control. The model is validated for biaxial failure of rock in compression and replicates typical pre- and post-peak strength metrics of stress drop, AE event counts, permeability evolution and failure modes. The model is applied to the simulation of hydraulic fracturing in permeable rocks to examine the effects of heterogeneities, permeability and borehole pressurization rate on the initiation of fracturing. The results indicate that more homogenous rocks require higher hydraulic pressure to initiate fracturing and breakdown. Moreover, both the fracturing initiation pressure and breakdown pressure decrease with permeability but increase with borehole pressurization rate, and the upper and lower limit of the initiation pressure are seen to be given by the impermeable (Hubbert–Willis) and permeable (Haimson–Fairhurst) borehole wall solutions, respectively. The numerical results are shown to be in good agreement with the experimental observations and theoretical results. This coupled damage and flow modeling approach provides an alternative way to solve a variety of complicated hydro-mechanical problems in practical rock engineering with the process coupling strictly enforced.  相似文献   

11.
Experimental studies were carried out in laboratory in order to investigate the effects of fracture on compressional (P) wave and shear (S) wave velocity propagation and therefore the relations between seismic properties and rock mass parameters. The discontinuity index, Id, fracture density parameter C, linear fracture parameter Γ and the rock quality designation (RQD) were used to describe the rock mass parameters. These parameters are analyzed and then related to the seismic properties. Four vertical aligned fractures were created on an intact calcarenite block, 0.6 m long, 0.4 m thick and 0.4 m width, by sawing. The measures were carried out in four different blocks of cacarenite, having the same physical properties, and in four different phases: in first block the fractures were filled with air; in the second block the fractures were filled with “terra rossa”; in third block the fractures were filled with wet “terra rossa” and in the fourth block the fractures were filled with clay. The test results were statistically analysed using the method of least squares regression and polynomial relationships with high correlation coefficient were found between the fractured rock parameters and P-wave, S-wave velocities and Vp/Vs ratio. The investigations suggest that the P-wave and S-wave velocities decrease with increasing the fracture parameters, while the Vp/Vs ratio increases with decreasing the fracture parameters.

Furthermore the results of the experimental studies were applied on the seismic refraction tomography data acquired in a great measurements campaign undertaken in the Adriatic salentina coast (south Italy) in order to monitor the coastal erosion.

The geophysical results, using the polynomial relationships between the fractured rock parameters and P-wave velocity, are in good agreement with the geomorphological and geological results.  相似文献   


12.
Fluid flow patterns have been determined using oxygen isotope isopleths in the Val-d’Or orogenic gold district. 3D numerical modelling of fluid flow and oxygen isotope exchange in the vein field shows that the fluid flow patterns can be reproduced if the lower boundary of the model is permeable, which represents middle or lower crustal rocks that are infiltrated by a metamorphic fluid generated at deeper levels. This boundary condition implies that the major crustal faults so conspicuous in vein fields do not act as the only major channel for upward fluid flow. The upper model boundary is impermeable except along the trace of major crustal faults where fluids are allowed to drain out of the vein field. This upper impermeable boundary condition represents a low-permeability layer in the crust that separates the overpressured fluid from the overlying hydrostatic fluid pressure regime. We propose that the role of major crustal faults in overpressured vein fields, independent of tectonic setting, is to drain hydrothermal fluids out of the vein field along a breach across an impermeable layer higher in the crust and above the vein field. This breach is crucial to allow flow out of the vein field and accumulation of metals in the fractures, and this breach has major implications for exploration for mineral resources. We propose that tectonic events that cause episodic metamorphic dehydration create a short-lived pulse of metamorphic fluid to rise along zones of transient permeability. This results in a fluid wave that propagates upward carrying metals to the mineralized area. Earthquakes along crustal shear zones cause dilation near jogs that draw fluids and deposit metals in an interconnected network of subsidiary shear zones. Fluid flow is arrested by an impermeable barrier separating the hydrostatic and lithostatic fluid pressure regimes. Fluids flow through the evolving and interconnected network of shear zones and by advection through the rock matrix. Episodic breaches in the impermeable barrier along the crustal shear zones allow fluid flow out of the vein field.  相似文献   

13.
低渗岩石非线性渗流机理讨论   总被引：12，自引：0，他引：12  
回顾了单相液体在低渗透饱和岩石的非线性渗流研究某些进展。从岩石孔隙结构、岩石与流体之间的相互作用和流体性质方面介绍了对低速非线性渗流产生机理的不同观点,探讨了孔隙大小、孔喉对启动压力的影响,讨论了界面张力、边界层、耦合渗流、非牛顿流体对非线性渗流形成的作用机制,并对进一步开展低渗透岩石渗流研究提出了建议。  相似文献   

14.
One-dimensional advection-dispersion models predict that characteristic δ18O vs. distance and δ18O vs. δ13C profiles should be produced during isothermal metamorphic fluid flow under equilibrium conditions. However, the patterns of isotopic resetting in rocks that have experienced fluid flow are often different from the predictions. Two-dimensional advection-dispersion simulations in systems with simple geometries suggest that such differences may be as a result of fluid channelling and need not indicate disequilibrium, high dispersivities, or polythermal flow. The patterns of isotopic resetting are a function of: (1) the permeability contrast between more permeable layers ('channels') and less permeable layers ('matrix'); (2) the width and spacing of the channels; (3) the width and spacing of discrete fractures; and (4) the orientation of the pressure gradient with respect to layering. In fractured systems, the efficiency of isotopic transport depends on the fracture aperture and the permeability of the surrounding rock. Resetting initially occurs along and immediately adjacent to the fractures, but with time isotopic resetting because of flow through the rock as a whole increases in importance. Application of the one-dimensional advection-dispersion equations to metamorphic fluid flow systems may yield incorrect estimates of fluid fluxes, intrinsic permeabilities, dispersivities, and permeability contrasts unless fluid flow occurred through zones of high permeability that were separated by relatively impermeable layers.  相似文献   

15.
为了研究煤岩水力压裂的起裂压力和水力压裂裂缝扩展规律,采用型煤试样,利用自主研发的水力压裂实验系统,参照现场压裂施工制定了“施加三向应力-顶部注水”的煤岩水力压裂物理模拟实验方案并开展了水力压裂实验,分析了不同条件下泵注压力和水力压裂裂缝。实验结果表明:压裂液泵注排量越大,起裂压力越大。三向应力满足最大水平主应力σH > 垂向应力σv > 最小水平主应力σh,水力压裂裂缝沿着垂直于σh的方向扩展。σv和σh一定,随着σH的增大,煤岩起裂压力先增大后减小,水力压裂裂缝扩展路径越平直。当σH远大于σv和σh时,水力压裂裂缝扩展路径越复杂,分叉缝角度越大。研究结果可为煤岩水力压裂理论的完善提供一定的参考和借鉴。   相似文献   

16.
A hypothetical model is proposed to explain the origin of compositionaldiscontinuities in the layering observed in orogenic lherzolites.The observed collinearity of the whole-rock peridotite compositionsis best explained in terms of partial melting and melt segregation.The presence of chemical discontinuities implies that melt segregationincludes an abrupt and discontinuous process. A key conceptin the model is the topological transformation of melt geometryin partially molten rocks responding to the equality and inequalityof the fluid pressure and solid pressure, which may be realizedin a gravitational field. It is emphasized that the percolationthreshold is a critical boundary, beyond which a rapid microstructuralchange occurs in response to the change of local fluid pressure,thus causing a rapid increase of permeability. The model impliesthat the mode of melting is closer to batch melting than tofractional melting in the upper mantle. KEY WORDS: critical phenomenon; partial melting; percolation threshold; Horoman peridotite; melt segregation  相似文献   

17.
Progressive vapour‐absent partial melting of a closed rock system increases melt pressure due to an expansion in the volume of the mineral plus melt assemblage. For a locally closed system, we quantify the melt pressure increase per increment of partial melting of a metapelite using phase equilibria modelling and combine it with Mohr–Coulomb theory to examine the interplay between melt pressure and fracture behaviour. It is shown that very small increments of vapour‐absent partial melting (<1%) increase melt pore pressure by tens of MPa leading to inevitable brittle failure of locally closed systems. Fracturing will affect these systems, even if initially limited to the scale of a few grains, and a connected microfracture network will enhance permeability as partial melting progresses. This will lead to a conditionally open system, potentially limiting accumulation of melt in the source. Repeated and cyclic fracture as temperature progressively increases will drive migration of the melt into sites of low fluid pressure at all scales. Crystal‐plastic creep processes create deformation‐induced dilatancy gradients that dominate over buoyancy forces at all scales in the melt source. Brittle and ductile deformation therefore cooperate in the extraction of melt. Enhanced porosity and permeability in ductile shear zones result in lower fluid pressure, providing a potentially important driving force for melt migration and drainage ‘up’ shear zones and along larger scale fluid pressure gradients in the crust.  相似文献   

18.
岩石变形破坏过程中渗透率演化规律的试验研究   总被引：12，自引：5，他引：7  
王环玲  徐卫亚  杨圣奇 《岩土力学》2006,27(10):1703-1708
利用伺服试验机对灰岩和砂岩进行了应力应变全过程渗透性试验,研究了岩样变形和破坏过程中的轴向应变与渗透率之间的关系,分析了岩样环向应变对渗透率的影响规律,探讨了岩样变形破坏前后渗透压差随时间的变化关系。结果表明,岩样渗透率与应力状态密切相关,渗透率的峰值滞后或超前于应力应变峰值,这与岩石介质本身的特性有关;渗透率-环向应变曲线与渗透率-轴向应变曲线有相同的变化趋势,但岩石环向变形比轴向变形更能灵敏地反映渗透率的演化规律;岩样变形破坏峰值前后的渗透压差与时间均遵循负指数关系。最后对岩石变形破坏过程中的渗透机理作了讨论。  相似文献   

19.
The high-temperature and high-pressure experiment on natural block rock indicates that dehydration-melting of hydrous biotite (Bi) and partial melting of felsic minerals in garnet-biotite-plagioclase gneiss are mainly controlled by temperature, while mineral phase transformation is not only controlled by temperature-pressure conditions but also genetically associated with hydrous mineral dehydration-melting and partial melting of felsic minerals. According to the characteristics of biotite dehydration-melting and garnet transformation reaction, three stages may be distinguished: (1) when the experimental temperature is 700℃, biotite transforms to ilmenite (Ilm) + magnetite (Mt) + H2O and garnet to magnetite (Mt); (2) when the temperature is 730-760℃, biotite is dehydrated and melted and transformed into K2O-rich melt + Ilm + Mt, and garnet, into hypersthene (Hy) + cordierite (Crd); (3) when the temperature is up to or higher than 790℃, biotite is dehydrated and melted and transformed into melt + Hy +  相似文献   

20.
Melting triggered by influx of a free aqueous fluid in the continental crust has commonly been inferred, but the source of water in such contexts remains a matter of debate. We focus on the Tertiary migmatites in the Southern Steep Belt of the Central Alps (Switzerland) to discuss the petrology, structures and geodynamic setting of water-assisted melting. These migmatites comprise various structural types (e.g. metatexites, diatexites, melt in shear zones), which reflect variable leucosome fractions. The melting event itself as well as the variable melt fractions are related to the amount of aqueous fluids. At a given P and T, melt-fractions in rocks of minimum melt composition correlate with the amount of infiltrated aqueous fluids. In more granodioritic systems the water distributes between melt and newly crystallizing hydrous phases such as amphibole, such that the melt fraction correlates with the contents of H2O, Al, and Ca in the system. Phase-equilibrium modelling indicates that the stabilization of amphibole leads to slightly lower melt fractions than in a granitic system at the same P, T and bulk water content. Phase-equilibrium models further indicate that in the Alpine migmatite belt: (1) several wt.% water (fluid:rock ratio of  1:30) are necessary to produce the inferred melt fraction; (2) the activity of H2O in the fluid is high; and (3) spatially associated metapelites are unlikely as a source for the required aqueous fluids.

We present a tectonic scenario for the southern margin of the Central Alps, to which these migmatites are confined, and we propose that water was produced from dehydration reactions in metapelites in the Southern Alps. We model fluid production rates at the time of melting and demonstrate that the resulting fluid flow pattern is mainly controlled by the differences in permeability between the fluid source region and melting region. The proposed model requires strong gradients in temperature and permeability for the two tectonic blocks. This is consistent with the scenario involving indenter tectonics at the boundary between the Central and the Southern Alps in Oligocene times.  相似文献   


设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号